Keep crossing those fingers and toes.

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National Zoo veterinarians believe they detected a developing panda fetus Wednesday morning during an ultrasound of giant panda Mei Xiang.

The fetus is about four centimeters -- which means Mei could give birth any time between now and early September, the National Zoo said.

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"There might have been a little bit of dancing" when vets saw the ultrasound, a zoo scientist said during a live interview via Periscope late Wednesday afternoon. He said keepers are hoping for a healthy cub.

At this stage, vets typically only detect changes in Mei Xiang's uterus, which happens in both pregnancy and pseudo (false) pregnancy. Mei's previous cubs were never detected via ultrasound.

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Additionally, Mei is typically not very cooperative at this stage of pregnancy or pseudopregnancy. Much to everyone's surprise, she responded to the panda's keepers calls Wednesday morning.

"Today, we are cautiously optimistic," said Dennis Kelly, director of the Smithsonian's National Zoo, in a release Wednesday. "We want a healthy cub for all the right conservation reasons. I am excited, but I have to say that we were prepared for a cub even before this morning's ultrasound... I'll just ask everyone to remain positive with us."

There is still a chance that Mei could reabsorb or miscarry a fetus, the zoo said.

Mei was inseminated April 26 and 27 with frozen sperm from Hui Hui, a panda living in China, and with sperm from the zoo's male giant panda, Tian Tian, the Smithsonian Conservation Biology Institute said.

On Aug. 10, Mei's hormone levels were on the rise as she began to show typical signs of pregnancy or pseudopregnancy, including spending more time in her den, sleeping more, body licking and cradling objects.

If a healthy cub is born, it would be the first time the National Zoo had a newborn and an older cub -- in this case, 2-year-old Bao Bao -- at the same time.

Mei Xiang has previously given birth to four cubs, two of whom survived: male Tai Shan, who was born at the zoo in 2005 and now lives in China, and female Bao Bao, who still lives at the National Zoo. A female cub born in 2012 died a week after birth. Another female cub, Bao Bao's twin, was stillborn.